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Polysaccharides and polypeptides are two types of biopolymers that are used in biomedical, industrial, and commercial applications. Both families of biopolymers are generally biodegradable, sustainable, and often exhibit low toxicity. Polysaccharides and polypeptides are polymers derived from natural resources and can be modified or synthesized through polymerization of various monomers. Polypeptides, specifically, are typically synthesized by polymerizing monomers such as N-carboxyanhydrides (NCAs) or N-thiocarboxyanhydrides (NTAs) to form homopolymers or random copolymers when using two different NCA/NTA monomers simultaneously. Chapter 1 begins with a background on polysaccharide and polypeptide-based materials with a focus on polysaccharide-block-polypeptide block copolymers. Previous work includes combining these two biopolymers through methods requiring post-polymerization purification. Chapter 1 introduces the field, challenges it faces, and how this work can help pose some solutions to these challenges. In this thesis, we utilized NTAs to synthesize polypeptides (Chapters 2 and 3) and as an H2S donor (Chapter 4). Combining polysaccharides and polypeptides into a block copolymer is useful for drug delivery and blend compatibilization applications. In Chapter 2, we synthesized a dextran-block-poly(benzyl glutamate) block copolymer that is amphiphilic; the differences in hydrophilicity among the two blocks allowed for nanostructures to form in situ in water, which we envision can be used for applications in drug delivery. Because nanostructures are formed in situ, this method negates the need for post-polymerization modification or purification, a requirement of many other nanostructure formation procedures. Coarse-grained molecular dynamics simulations were employed to shed light on interactions found on the molecular level. The interactions studied were then used to explain the nanostructures observed experimentally. In Chapter 3, we similarly formed another polysaccharide-block-polypeptide with the same poly(benzyl glutamate) polypeptide used in Chapter 2 but using ethyl cellulose for the polysaccharide. Poly(benzyl glutamate) is similar in structure to the commercial plastic polyethylene terephthalate (PET), a petroleum-based polymer that is not biodegradable. Therefore, this ethyl cellulose-block-poly(benzyl glutamate) BCP was used as compatibilizer to improve mixing in immiscible ethyl cellulose/PET blends. These blends afforded a more bio-derived alternative to PET/petroleum-based plastics. This chapter focuses on the synthetic efforts, a common challenge with polysaccharides, to produce this block copolymer as well as blend preparation and characterization. Chapter 4 utilizes an NTA as an H2S donor rather than a monomer for polymerization. H2S is an endogenous signaling gas that plays an important role in many organs and systems. In humans, H2S deficiency leads to a range of medical issues including hypertension, preeclampsia, liver diseases, and Alzheimer's disease. NTAs are advantageous for H2S delivery in the biomedical field due to their amino-acid origin and innocuous byproducts. The NTA donor in this work was attached to amylopectin via thiol-ene "click" photochemistry with the amino acid cysteine providing the thiol source on amylopectin. H2S release half-lives were in the range of several hours and depended on polymer molecular weight. Lastly, Chapter 5 summarizes the conclusions formed from these projects as well as potential future extensions from this work. / Doctor of Philosophy / Polysaccharides, long-chain sugars, and polypeptides, long-chain amino acid sequences, are two types of biopolymers that are used in biomedical, industrial, and other commercial applications. Both families of biopolymers are generally biodegradable, sustainable, and often exhibit low toxicity. Chapter 1 begins with a background on polysaccharide and polypeptide-based materials with a focus on polysaccharide-block-polypeptide block copolymers. Chapter 1 introduces the field, challenges it faces, and how this work can help pose some solutions to these challenges. In this thesis, we utilized N-thiocarboxyanhydrides (NTAs) to synthesize polypeptides (Chapters 2 and 3) and as an H2S donor (Chapter 4). In Chapter 2, we synthesized dextran-block-poly(benzyl glutamate), a polysaccharide-block-polypeptide block copolymer, that is both hydrophilic and hydrophobic. The differences in hydrophilicity among the two blocks allowed for nanostructures to form in situ in water, which we envision can be used for applications in drug delivery. Computational modeling was then employed to help explain the nanostructures observed experimentally. In Chapter 3, we similarly formed another type of polysaccharide-block-polypeptide. The polypeptide used is similar in structure to the commercial plastic polyethylene terephthalate (PET), a petroleum-based polyester that is not biodegradable. This block copolymer was then employed to improve mixing between blends of immiscible ethyl cellulose (polysaccharide) and PET. These blends afford a more bio-derived alternative to PET/petroleum-based plastics. This chapter focuses on the synthetic efforts, a common challenge with polysaccharides, to produce this block copolymer as well as blend preparation and characterization. Chapter 4 utilizes an NTA as an H2S donor rather than a monomer for polymerization. H2S is an endogenous signaling gas that plays an important signaling role in many organs and systems. In humans, H2S deficiency leads to a range of medical issues including hypertension, preeclampsia, liver diseases, and Alzheimer's disease. In this work, we synthesized a polymeric polysaccharide H2S donor with tunable release rates, which is beneficial for longer therapeutic time and increased patient compliance. Lastly, Chapter 5 summarizes the conclusions formed from these projects as well as potential future extensions from this work.

Well-defined central functionalized asymmetric tri-block copolymers (CFABC) were designed as a new type of polymer brush surface modifier, with a short central functionalized block that can form chemical bonds with a suitable substrate surface. A combination of sequential living anionic polymerization and polymer modification reactions were used for the synthesis of two CFABCs: PS-b-poly(4-hydroxystyrene)-b-PMMA and PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA. GPC and NMR characterization indicated that the block copolymers possessed controlled molecular weights and narrow molecular weight distributions. CFABC polymer brushes were successfully prepared by chemically grafting PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA onto silicon wafer surfaces. AFM, XPS and ellipsometry were used to confirm the CFABC polymer brush structures and thickness. The surface properties of CFABC polymer brush modified silicon wafer substrates subjected to different environmental parameters were studied. Reversibly switchable surface energies were observed when the polymer brush modified surfaces were exposed to solvents with different polarities. The phenomenon was attributed to the chain configuration auto-adjustment in the polymer brush systems. The same mechanism was also used to explain the enhanced adhesion capability between the modified surfaces and different polymer materials (PS and PMMA). Phase behaviors of polymer thin films on unmodified and CFABC polymer brush modified silicon wafer surfaces were also studied. For thin films of polymer blends, PS blend PS-co-PMMA, the effects of film thickness, chemical composition and temperature on the phase separation mechanism were investigated. The phase behavior in thin films of triblock copolymers with or without central functionalities were compared to reveal the role of the central functionalized groups in controlling film structures. Finally, the presence of CFABC polymer brush at the interface between PS-b-PMMA diblock copolymer thin film and silicon wafer substrate was found to decrease the characteristic lamellar thickness in the thin film. A mechanism of tilted chain configurations in the thin film due to the interactions with the CFABC polymer brushes was proposed. / Ph. D.

The major challenge of the research was to characterize and develop concepts for establishing structure/property relationships between the functionality of the polymer backbone, the states of water and the membrane transport properties. Most of the hydrocarbon based random copolymers reported in the literature show reduced proton conductivity at low water content. This was attributed to the formation of an isolated morphology. Over the last few years our group has synthesized thermally stable multiblock copolymers with varying chemical structures and compositions. Block copolymers consist of two or more incompatible polymers (i.e. blocks) that are chemically conjoined in the same chain. The transport properties of the multiblock copolymers showed a strong dependence on the morphology in contrast to the random copolymers. Irrespective of the nature of the backbone, the transport properties scaled with the block lengths of the copolymers. An increase in block length for a given series of block copolymer was associated with improved proton conduction, particularly under partially hydrated conditions compared to the random copolymers. The structure-property relationship of the proton conductivity and self-diffusion coefficient of water was obtained as a function of the volume fraction of water for all the random and block copolymers. At a given volume fraction, the block copolymers displayed both higher self-diffusion coefficients of water and proton conductivities relative to the random copolymers. This improvement in transport properties indicates the presence of desired and favorable morphology for the blocks. For DMFC applications, the block copolymers also showed low methanol permeability and high selectivity. The states of water in the copolymers were characterized using DSC and NMR relaxation techniques. At similar ionic contents, the free water concentration increased with increasing block lengths. The distribution of the states of water in the copolymers correlates to transport properties. This knowledge, coupled with the state of water experiments, transport measurements, and chemical structure of the copolymers provided a fundamental picture of how the chemical nature of a phase separated copolymer influences its transport properties. The experimental procedure involved impedance spectroscopy, DSC, TGA, FTIR, DMA, pulse gradient stimulated echo (PGSE) NMR, NMR relaxation experiments and various electrochemical fuel cell performance experiments. / Ph. D.

This dissertation is part of an effort to understand and to facilitate the modeling of the ordering kinetics of block copolymers in solution during the extraction of solvent from a solution-cast film. Central to this work was determining a suitable method for measuring the ordering kinetics during solvent removal and being able to interpret the measurements in terms of structure development. It was also necessary to assess a model for quantifying the ordering kinetics to use in conjunction with a mass transfer model to predict structure formation during solvent extraction. Changes in the dynamic mechanical response (DMR) over time of block copolymer solutions at fixed concentrations following solvent removal were explored as a means to track the growth of ordered domains. It was found that DMR measurements performed following solvent extraction were sensitive to the nucleation and growth process of the phase separation process over a wide range of concentrations, beginning near the order-disorder transition concentration. Based on complimentary small angle X-ray measurements, it was determined that the changes in the DMR are caused by the development of individual microstructures, The SAXS experiments also indicated that the DMR is insensitive to late stages of the growth process. Ultimately, DMR measurements under-predicted the ordering times at several concentrations and did not detect ordering at concentrations above which SAXS data indicated ordering was still occurring. The ability to use the parallel and series rules of mixtures for determining ï ¦(t) in conjunction with the Avrami equation to quantitatively model the ordering kinetics was also determined. These models allowed the ordering kinetics during solvent removal to be qualitatively analyzed. However, using the two different rules of mixtures resulted in a wide range of possible ordering times for a given copolymer concentration, making these approximations unsuitable for modeling a real solvent extraction process. Further, the parameters of the model were insensitive to the type of microstructures developing. As a continuation of this work, a new apparatus to track block copolymer ordering in situ during solvent extraction was designed. Experiments using the apparatus allowed the ordering kinetics and domain dimensions as a function of concentration to be monitored in real-time under several solvent removal conditions. These experiments study the ordering kinetics is a manner more akin to real processing conditions and will allow future assessment of the ability of iso-concentration ordering kinetics to predict phase separation during film processing. / Ph. D.

Poly(arylene ether)s are a well-established class of thermoplastics that are known for their mechanical toughness, thermal stability, and fabrication into membranes. These materials can undergo a myriad of modifications including backbone structure variability, sulfonation, and crosslinking. In this dissertation, structure-property relationships are considered for poly(arylene ether)s with regard to membrane applications for proton exchange and gas separation membranes. All of the proton exchange membranes in this dissertation focus on a disulfonated poly(arylene ether sulfone) based hydrophilic structure to produce hydrophilic-hydrophobic multiblock copolymers. The hydrophobic segments were based upon poly(arylene ether benzonitrile) polymers and copolymers. The oligomers were synthesized and isolated separately, then reacted under mild conditions to form the alternating multiblock copolymers. Structure-property relationships were considered for two different proton exchange membrane applications. One multiblock copolymer system was for H2/air fuel cells, and the other for direct methanol fuel cells (DMFCs). The H2/air fuel cells operate under harsh conditions and varying levels of relative humidity, while the DMFCs operate in an aqueous environment with a methanol-water mixture (typically 0.5-1 M MeOH). Thus two different approaches were taken for the multiblock copolymers. All of the multiblock copolymers were cast into membranes and after annealing resulted in drastically reduced water uptake as compared to random and non-annealed systems. The membranes were characterized with regard to composition, mechanical properties, morphology, water uptake, proton conductivity, and molecular weight. Membranes were also sent to collaborators to elicit the fuel cell performance of the proton exchange membranes. In H2/air fuel cells the approach was to increase charge density by bisphenol choice in the hydrophilic phase. This was performed by switching to a lower molecular weight monomer, hydroquinone, and a monosulfonated hydroquinone. This produced higher charge density in the hydrophilic phase, and the corresponding multiblock copolymer. With increased hydrophilicity the multiblock copolymers showed increased phase separation, proton conductivity, and better performance under relative humidity testing. In the second system for DMFCs, the primary goal was to reduce methanol permeability by bisphenol selection in the hydrophobic phase. This was done with by replacing fifty mole percent of the fluorinated monomer with a series of increasing hydrophobicity bisphenols. Addition of benzylic methyl groups on the bisphenols, was the method undertaken to increase the hydrophobicity. The combination of reduced fluorine content along with the addition of methyl groups resulted in multiblock copolymers with extremely low water uptake and methanol permeability. This allowed for a PEM with better performance than Nafion® in 1M MeOH in DMFC testing. The gas separation membranes presented in this dissertation are based upon poly(arylene ether ketone)s. Two systems were presented: one with a polymer directly synthesized with a bisphenol containing benzylic methyl groups and 4,4'-difluorobenzophenone, and the other a difunctional poly(phenylene oxide) oligomer polymerized with 4,4'-difluorobenzophenone. These systems were crosslinked via UV light through excitation of the ketone group to the triplet state and then hydrogen abstraction from the benzylic methyl. Confirmation of crosslinking was performed via differential scanning calorimetry and infrared spectroscopy. Changes in the glass transitions between crosslinked and non-crosslinked materials were characterized with respect to the concentration of ketones to elicit the effects of crosslink density on the polymers and copolymers. Gas transport properties showed a strong dependence on the ketone percentage as the selectivity was much higher for the homopolymer, while the permeability was higher for the PPO copolymer in the CO2/CH4 and O2/N2 gas pairs. / Ph. D.